This invention relates to the chemical arts. In particular it relates to a method for the preparation of a cross-linked polymer of dicyclopentadiene (hereinafter referred to as DCPD) with a metathesis-catalyst system.
Cross-linked poly(DCPD) is well suited for a wide variety of applications, particularly as an engineering plastic or foam. The number of applications has been somewhat limited, however, because of the dark color and cloudy appearance of the polymer. Consequently, there is a desire to produce cross-linked poly(DCPD) which has a light color and good transparency.
One of the great advantages offered by poly(DCPD) is that the catalysis of DCPD monomer can be controlled so that a reaction mixture can be formed in one place and then the reaction mixture injected to a second place where polymerization occurs. A prime example of how this advantage is put to use is reaction injection molding (sometimes hereinafter referred to as RIM). Reaction injection molding is a process for inmold polymerization which involves the mixing of two or more low viscosity reactant streams in a mixing head. The combined streams are then injected into a mold where they quickly react to form a solid infusible mass. For a particular catalyst system to be of use with RIM and related processes, certain requirements must be met:
(1) the individual reactant streams must be stable and must have a reasonable shelf-life under ambient conditions,
(2) it must be possible to combine the reactant streams without their reacting in the mixing head;
(3) when injected into the mold, the materials must react rapidly to form a solid; and
(4) any additives incorporated into the catalyst system must not interfere with the above requirements. Where a cellular cross-linked polymer is the desired product an additional requirement must be met:
(5) the reactant streams must set up into a cellular cross-linked structure.
It can be seen that in RIM or related processes the catalyst is critical. For a process to be of use in commercial situations the polymerization should occur quickly, but the polymerization cannot occur so quickly that the cross-linked polymer is formed before the combined reactant streams can be injected into the mold. Moreover, in those situations where a cross-linked polymeric foam is desired, the foaming (the formation of bubbles and their subsequent expansion) must occur before the combined reactant streams begin to cross-link to any significant degree, but the foaming must occur after the combined reactant streams have developed a viscosity sufficient to support a foamed structure.
It is known in the prior art to base a metathesis-catalyst system useful in polymerization of cycloolefins on a pentavalent tantalum catalyst and an organoaluminum or organoaluminum halide activator. For example, U.S. Pat. No. 3,492,278 discloses the preparation of an essentially gel free elastomer from cyclopentadiene using, inter alia, tantalum pentahalide. U.S. Pat. No. 3,707,520 discloses the preparation of linear elastomers from cyclopentadiene and up to 20% by weight of a diolefin, including dicyclopentadiene, using, inter alia, tantalum pentachloride and organoaluminum or an organoaluminum halide activator.
U.S. Pat. 4,400,340 discloses the use of a metathesiscatalyst; preferably a tungsten containing metathesis-catalyst; an organoaluminum or organoaluminum halide activator; and a rate moderator and the use of a RIM process to form a cross-linked poly(DCPD).
In accordance with the invention a method is provided for preparing a cross-linked polymer having light color and good transparency comprising polymerized units of DCPD employing a metathesis-catalyst system which does not require a rate modifier.